Rotaviruses, members of the family Reoviridae, are the most important cause of acute dehydrating diarrhea in infants and young children and on an annual basis are responsible for more than 500,000 deaths worldwide. Their impact on human morbidity and mortality has stimulated extensive efforts towards the development of rotavirus vaccines by the Laboratory of Infectious Diseases (LID). Large scale phase III trials have shown that a rhesus-based rotavirus tetravalent vaccine developed by the LID is highly effective in protecting against the severe and life-threatening dehydrating diarrhea caused by these viruses. The tetravalent vaccine was approved for use in humans by the FDA, marketed by Wyeth-Lederle as Rotashield, and administered to approximately one million children. However, because of the possibility that the vaccine may infrequently induce intussusception. the use of the vaccine has been discontinued at least for the present. Due to the great impact that rotavirus illness continues to have on human mortality and morbidity, it remains a goal of the Laboratory to develop vaccines against these agents that are effective and safe. The initial phase of this project focuses on the development of a reverse genetics system that can be used to alter the genetic information of the rotavirus genome. Subsequently, this system will be used as a tool to identify loci within the genome that defines the growth characteristics, antigenic properties, and virulence of rotaviruses. In the final phase of the project, the reverse genetics system will be used to create a new generation of potentially more effective vaccines by the introduction of attenuating mutations into the genome of virulent isolates of human rotaviruses. The reverse genetics system can also be applied to improving the growth characteristics of the vaccine viruses in cell culture, to increasing the immunogenicity of the vaccine, and to altering the antigenicity of the vaccine to include non-G1-4 serotypes.